The ability of phorbol ester tumor promoters to alter the proliferation and differentiation of many cell types is well documented. The molecular mechanisms involved are obscure. Because these or other environmental chemicals with similar properties probably contribute significantly to the development of human neoplasias, understanding their mechanism of action at the cellular level is important for designing appropriate treatment and prevention strategies. The goal of these studies is to relate rapid changes in monovalent ion transport caused by phorbol esters to longer term changes in cellular behavior, especially the onset of cell proliferation. The hypothesis that these agents increase intracellular K+ levels by a possibly unique mechanism, via inhibition of an ouabain-insensitive furosemide-sensitive Na+K+C1- cotransport system, will be tested in several cell lines in culture, including BALB/c 3T3 preadipose cells, normal human fibroblasts, and HL-60 human leukemic cells. Similar experiments will be carried out in epithelial tissues and cell cultures, including rodent primary epidermal cells and epidermal cell lines. The mechanism by which phorbol esters reduce cell volume will be explored, and the relationship between volume changes and initiation of cell proliferation in quescent cells will be studied. The involvement of phorbol ester receptors in mediating changes in ion fluxes will be critically assessed. These studied will determine whether rapid perturbations in cellular ionic homeostasis are important in phorbol ester-induced biologic changes in cell culture.